Field of the Invention
The present invention relates to measuring system and a method for operating a measuring system for detection of wire breakage in measuring systems comprising at least one first sensor, which sensor is connected to a measuring system by at least two wires, which measuring system comprises a current source, which current source is generating a current in the wires and through at least one first sensor which current is detected in the measuring system, which measuring system based on the measured current or voltage measures the impedance of at least the first sensor which measuring system transmit the result of the measurement of the impedance to a connected system.
Description of Related Art
In basic temperature measurements based on either RTD or TC, some of the wires used for determining the resistance of the RTD or the voltage across the TC, will be current less and can therefore in case of a wire breakage contribute to an erroneous measurement.
Wire breakage detection is well known within the temperature transmitter industry and several patents describe different technologies:
Japanese Patent JP55060828 “Detector for thermocouple failure” describes a technology, where both a differential current and a common mode current are fed into the TC sensor to analyze for insulation deterioration and wire breakage. During the test, the measurement of the TC voltage, and thereby the temperature, is invalid.
Japanese Patent JP59065267 “Apparatus for detecting wire breakage of temperature detecting element” describes a technology, where two RTD's are used to evaluate the existence of wire breakage within one of the two RTD sensors. And the technology does not support compensation for wire resistance.
Japanese Patent JP2011242349 “Resistance temperature detector circuit” describes a technology for 3-wire RTD measurements, which includes a wire breakage detection based on a change in the current fed to the RTD. As the current changes all voltages levels will change accordingly and thereby an indication of a wire breakage can be detected, if two consecutive measurements at different RTD sensor currents shows different RTD values.
Temperature transmitters are generally implemented with a wire breakage detection system based on periodically injection of a test current through the non-conducting wires followed by a test of the resulting voltage. A wire breakage will cause the voltage to increase. During the injection of the test current and a relaxing period afterwards the temperature measurement is disabled as the measurement will be influenced by the injection current and thereby not valid. This wire breakage detection method causes the temperature measurements to be relatively slow (rise time in the range of 0.5 to 1 s).